Characterization And Fabrication Of Lead-free Perovskite

Solar energy is the ultimate energy resources for the earth.As we keep researching on solar energy,technology of solar cells are developed.Also there are many good materials including perovskite.The metal halide perovskite is a new type of semiconductors which has many unique optical and electronic properties,such as a broad absorption spectrum,an adjustable band gap,high carrier mobility,and long diffusion length.These characteristics attract researchers all around the world to study their applications in the field of photovoltaic and optoelectronics.However,this ABX3-structure material is basic on toxic element lead.Although this material shows amazing performance,we still need to find another material to replace it.On one hand,while tin perovskite is more sensitive to atmosphere and difficult to fabricate,we still developed a new method to fabricate lead-free perovskite by thermal evaporation and solvent annealing.We first deposit SnI2 and CH3NH3I separately on substrate.While bilayer film forms,put the film under solvent atmosphere to accelerate reaction between these precursors.Perovskite structure will form after includes old bond breaking and new bond forming under annealing temperature.However,we should keep suitable annealing temperature and solvent concentration.If annealing temperature is too high,solvent may volatilize fast which may cause incomplete conversion.If solvent concentration is too high,solvent will dissolve bilayer film,which may cause island type of perovskite for fast crystallization rate of tin-based perovskite.Keeping balance between these two factors can help us achieve perovskite with good coverage,high quality and big crystal.Devices based on this film show amazing performance which is over 4%efficiency.Also,compared to solution method,evaporation can be used in large-scale production for better crystal quality and morphology.On the other hand,we are still looking for element which is in the same cycle of Lead because the most peripheral electronic structure of lead ions play an important role in photoelectric properties.However,thallium and bismuth ions have different chemical valance.As we can see,these elements which replace lead will cause holes when perovskite forms.So we should find another element which can keep in balance.Considering exciton binding energy,decomposition enthalpy,effective mass of holes and electrons and so on,we find that bismuth and silver would be ideal candidate elements,which provide good stability of chemical valence state and show good photoelectric properties.Through tuning ratio of halogen,we can achieve suitable 1.89 eV bandgap perovskite.In summary,through analysis difference between lead-based and tin-based perovskite,we replace the traditional solution method with evaporation and solvent annealing.We can achieve high quality perovskite by this new method.We believe this invention will help improve tin-based perovskite.At the same time,stable and non-toxic bismuth-halide double perovskite show us that it will achieve better efficiency in the future.This kind of perovskite will be a good research direction.